Effects of donor unit and π-bridge on photovoltaic properties of D-A copolymers based on benzo[1,2-b:4,5-c']-dithiophene-4,8-dione acceptor unit

Article abstract of DOI:10.1002/pola.27209

A series of donor-π-acceptor (D-π-A) conjugated copolymers (PBDT-AT, PDTS-AT, PBDT-TT, and PDTS-TT), based on benzo[1,2-b:4,5-c']dithiophene-4,8- dione (BDD) acceptor unit with benzodithiophene (BDT) or dithienosilole (DTS) as donor unit, alkylthiophene (AT) or thieno[3,2-b]thiophene (TT) as conjugated π-bridge, were designed and synthesized for application as donor materials in polymer solar cells (PSCs). Effects of the donor unit and π-bridge on the optical and electrochemical properties, hole mobilities, and photovoltaic performance of the D-π-A copolymers were investigated. PSCs with the polymers as donor and PC₇₀BM as acceptor exhibit an initial power conversion efficiency (PCE) of 5.46% for PBDT-AT, 2.62% for PDTS-AT, 0.82% for PBDT-TT, and 2.38% for PDTS-TT. After methanol treatment, the PCE was increased up to 5.91%, 3.06%, 1.45%, and 2.45% for PBDT-AT, PDTS-AT, PBDT-TT, and PDTS-TT, respectively, with significantly increased FF. The effects of methanol treatment on the photovoltaic performance of the PSCs can be ascribed to the increased and balanced carrier transport and the formation of better nanoscaled interpenetrating network in the active layer. The results indicate that both donor unit and π-bridge are crucial in designing a D-π-A copolymer for high-performance photovoltaic materials.

Full text of DOI:10.1002/pola.27209

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Effects of Donor Unit and p-Bridge on Photovoltaic Properties
of D–A Copolymers Based on Benzo[1,2-b:4,5-c’]-dithiophene-4,8-dione
Acceptor Unit
Haijun Bin,1,2 Lu Xiao,2 Yong Liu,1 Ping Shen,1,2 Yongfang Li2
1College of Chemistry and Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education,
Xiangtan University, Xiangtan 411105, People’s Republic of China
2CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic
of China
Correspondence to: P. Shen (E-mail: shenping802002@163.com) or Y. Li (E-mail: liyf@iccas.ac.cn)
Received 14 February 2014; accepted 6 April 2014; published online 29 April 2014
DOI: 10.1002/pola.27209
ABSTRACT: A series of donor-p-acceptor (D-p-A) conjugated
copolymers (PBDT-AT, PDTS-AT, PBDT-TT, and PDTS-TT),
increased up to 5.91%, 3.06%, 1.45%, and 2.45% for PBDT-AT,
PDTS-AT, PBDT-TT, and PDTS-TT, respectively, with signifi-
cantly increased FF. The effects of methanol treatment on the
photovoltaic performance of the PSCs can be ascribed to the
increased and balanced carrier transport and the formation of
better nanoscaled interpenetrating network in the active layer.
The results indicate that both donor unit and p-bridge are cru-
based on benzo[1,2-b:4,5-c’]dithiophene-4,8-dione
(BDD)
acceptor unit with benzodithiophene (BDT) or dithienosilole
(DTS) as donor unit, alkylthiophene (AT) or thieno[3,2-b]thio-
phene (TT) as conjugated p-bridge, were designed and synthe-
sized for application as donor materials in polymer solar cells
(PSCs). Effects of the donor unit and p-bridge on the optical
and electrochemical properties, hole mobilities, and photovol-
taic performance of the D-p-A copolymers were investigated.
PSCs with the polymers as donor and PC70BM as acceptor
exhibit an initial power conversion efficiency (PCE) of 5.46%
for PBDT-AT, 2.62% for PDTS-AT, 0.82% for PBDT-TT, and
2.38% for PDTS-TT. After methanol treatment, the PCE was
cial in designing a D-p-A copolymer for high-performance pho-
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tovoltaic materials.
2014 Wiley Periodicals, Inc. J. Polym.
Sci., Part A: Polym. Chem. 2014, 52, 1929–1940
KEYWORDS: benzo[1,2-b:4,5-c0]-dithiophene-4,8-dione; p-bridge;
D-p-A copolymers; photovoltaic properties; polymer solar cells
rier mobilities by intramolecular charge transfer (ICT) from
donor to acceptor moieties.3 The power conversion efficien-
cies (PCEs) of the PSCs based on a number of D–A copoly-
mers have reached up to 7%–9%.4
INTRODUCTION During the past decade, polymer solar cells
(PSCs) have been extensively investigated owing to their
light weight, flexibility, and potentially inexpensive manufac-
turing.1 The most successful PSCs to date are based on the
bulk heterojunction (BHJ) devices formed from a blend of
polymeric electron donor and fullerene derivative electron
acceptor such as [6,6]-phenyl-C61-butyric acid methyl ester
(PC60BM) or [6,6]-phenyl-C71-butyric acid methyl ester
(PC70BM).2 A persistent challenge in the development of
polymer based donors has been fine-tuning of various struc-
tural and electronic features in order to maintain wide-
ranging optical absorption profiles, the appropriate energy
levels, high charge-carrier mobilities, solution processability,
large absorption coefficients, and a favorable blend morphol-
ogy.1 Low bandgap conjugated polymers with electron
donor–acceptor (D–A) architecture are particularly attractive,
because of the facile tunability of their absorption bands,
highest occupied molecular orbital (HOMO)/lowest unoccu-
pied molecular orbital (LUMO) energy levels, and charge car-
To further manipulate stereo-structure and optoelectronic
properties of conjugated copolymers, some p-conjugated
bridges, typically thiophene or alkylthiophene (AT) unit,
were inserted between donor (D) and acceptor (A) moieties
to produce D-p-A type conjugated copolymers.5 Many efforts
have been devoted to the design and synthesis of new D
and A building blocks to obtain efficient copolymers for
PSCs.3,4 But there are only a few studies that focus on
the p-conjugated bridges between
D and A
moieties.6
p-Conjugated bridges crucially influence the electronic struc-
ture of polymer main chain and the interaction between D
and
A units; therefore, they remarkably affect stereo-
structure and consequently photophysical, electrochemical,
charge transport, and photovoltaic properties of the D-p-A
Additional Supporting Information may be found in the online version of this article.
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FIGURE 1 Chemical structures of the four D-p-A copolymers. [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
conjugated copolymers. In fact, the three parts in D-p-A type
copolymers are very important in designing high-
performance polymer photovoltaic materials. Thus, it is of
prime importance to fully explore the effects of the donor
units along with p-bridges on the physicochemical and pho-
tovoltaic properties in the design of new polymers for effi-
cient PSCs. Recently, Li and Hou et al. reported a series of D-
p-A copolymer with different p-bridges.6a,e,f,g The results
showed that the p-bridges significantly affect the molecular
architecture and optoelectronic properties of the copolymers.
to build D-p-A copolymers. These copolymers showed high-
hole mobility in field-effect transistor8 and PCE over 5% in
PSCs.6f,g,i,8a The reason is TT had a rigid and coplanar fused
ring which ensures a highly delocalized p-electron system and
strong intermolecular p–p stacking.
As for the acceptor unit in the D–A copolymers, benzo[1,2-
b:4,5-c’]-dithiophene-4,8-dione (BDD)9 and its isomer
benzo[1,2-c:4,5-c’]dithiophene-4,8-dione10 unit have attracted
interests due to their strong electron-withdrawing ability
and fine chemical–structural modification. Some polymers
based on these two acceptor units exhibited attractive and
interesting properties in optoelectronic devices.9,10 Zhu et al.
reported low bandgap D–A copolymers with BDD as an
acceptor unit and BDT and DTS as donor units. PSCs based
on BDT–BDD copolymer gave a medium PCE of 2.18% due
to its relatively narrow absorption. However, the PSC based
on the DTS–BDD copolymer showed a good PCE up to
4.33% along with a high open-circuit voltage (Voc, ꢀ0.9 V),
indicating that BDD is a potential building block for highly
efficient conjugated polymer donors.9 The results inspire us
to insert a p-conjugated bridge between the BDT or DTS
donor unit and BDD acceptor unit, which could provide an
opportunity to tune the stereo-structure and optoelectronic
Benzodithiophene (BDT) and dithienosilole (DTS) are attrac-
tive for the donor units in D-p-A copolymers due to their good
electron-donating ability and planar molecular structure. The
D–A copolymers based on BDT or DTS donor unit show broad
absorption, relatively low-lying HOMO energy level, and high
hole mobility, which are desirable for obtaining high-
performance PSCs. Moreover, many researchers have demon-
strated that AT-substituted BDT unit is a better donor unit in
constructing high-performance copolymers in comparison
with its alkoxyl-substituted counterpart.7 In previous reports,
thiophene or AT units are mainly used as p-bridges in D-p-A
copolymers with excellent photovoltaic properties. Recently,
the thieno[3,2-b]thiophene (TT) unit was used as the p-bridge
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SCHEME 1 Synthetic routes of monomers and the four copolymers. [Color figure can be viewed in the online issue, which is available
at wileyonlinelibrary.com.]
properties of the resulting polymers, which is expected to
improve the photovoltaic performance.
brominated with bromine in acetic acid to give 2,5-dibromo-
thiophene-3,4-dicarboxylic acid (compound 1) in a good
yield (75%). Treating compound 1 with excess oxalyl chlo-
ride gave the intermediate 2,5-dibromothiophene-3,4-dicar-
bonyl dichloride (compound 2). This compound was directly
used in the next step without further purification. Then an
important compound 3 based on BDD was obtained by a
Friedel–Crafts reaction between compound 2 and 2-(2-ethyl-
hexyl)thiophene in presence of AlCl3 with 12.9% yield. Next,
the BDD-based compound reacted with AT and TT, respec-
tively, to afford compounds 4a and 4b via a Stille coupling
reaction. The bridges of AT and TT containing monomers M1
and M2 were produced by bromination of 4a and 4b with
NBS, respectively. The BDT- and DTS-based monomers (M3,
M4, and M5) were prepared according to the literature
methods.7b,11 The four D-p-A copolymers PBDT-AT, PDTS-
AT, PBDT-TT, and PDTS-TT were all synthesized by the
palladium-catalyzed Stille-coupling polymerization. Crude
copolymers were purified by extracting with methanol,
Based on the above consideration, here, we designed and
synthesized a series of new D-p-A copolymers (PBDT-AT,
PDTS-AT, PBDT-TT, and PDTS-TT), with BDT or DTS as
donor unit, AT or TT as p-bridge, and benzodithiophene-4,8-
dione (BDD) as acceptor unit, as shown in Figure 1. The aim
of inserting p-conjugated bridges between the D and A moi-
eties is to tune the stereo-structure and broaden the absorp-
tion of the resulting copolymers. The effects of the different
donor and p-bridge upon the optical, electrochemical proper-
ties, hole mobilities, and photovoltaic performance of the
resulting copolymers were investigated.
RESULTS AND DISCUSSION
Synthesis and Characterization
The chemical structures and synthetic routes of monomers
and the four D-p-A polymers are outlined in Figure 1 and
Scheme 1, respectively. Thiophene-3,4-dicarboxylic acid was
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TABLE 1 Molecular Weights and Thermal Properties of the Copolymers
Copolymer
Yield (%)
Mn (kDa)a
Mw (kDa)a
PDI
Td (ꢁC)b
PBDT-AT
PDTS-AT
PBDT-TT
PDTS-TT
80
40
69
35
17.9
21.6
4.5
103.2
34.0
6.2
5.76
1.57
1.38
1.94
295
325
350
294
8.7
16.7
a
b
Determined by GPC in THF based on polystyrene standards.
Decomposition temperature, determined by TGA in nitrogen, based
on 5% weight loss.
hexane, and chloroform, respectively. The chloroform solu-
tion was concentrated and the product re-precipitated in
methanol to obtain the resulting copolymers. Table 1 sum-
marized the polymerization results and thermal properties
of the copolymers.
demonstrated in the cases of poly(3-hexylthiophene) and
other D–A polymers that molecular weight has a significant
impact on optoelectronic properties.4a
Thermal properties of the polymers were determined by ther-
mogravimetric analysis (TGA) under nitrogen atmosphere at a
heating rate of 20 ꢁC min21. The four polymers had good ther-
mal stability with onset decomposition temperatures corre-
sponding to 5% weight loss (Td) at above 290 ꢁC, as shown in
Table 1. Obviously, the thermal stability of the polymers is
adequate for the applications in PSCs and other optoelectronic
devices.
The polymers can be readily dissolved in common organic
solvents, such as chloroform, tetrahydrofuran (THF), chloro-
benzene, and 1,2-dichlorobenzene at room temperature or
with a slight heating. The molecular weights and polydisper-
sity indices (PDIs) of the copolymers were determined by
gel permeation chromatography (GPC) analysis with a poly-
styrene standard calibration and THF as an eluent. The four
copolymers showed relatively big difference in the molecular
weights. PBDT-AT gave a high weight-average molecular
weight (Mw) of 103.2 kDa with a large PDI of 5.76 (Table 1).
However, PBDT-TT showed a low Mw of 6.2 kDa with a small
PDI of 1.38 (Table 1). It can be seen that the copolymers
(PBDT-TT and PDTS-TT) with TT as the conjugated bridge
have lower molecular weights than those of the copolymers
(PBDT-AT and PDTS-AT) with AT as the conjugated bridge,
which could result from the poorer solubility of the former.
One possible reason could be ascribed to the absence of
alkyl groups in the TT bridge. High molecular weight is
desirable because it will help to improve the film-forming
ability and photovoltaic performance. It has previously been
Optical Properties
Figure 2 shows absorption spectra of the four polymers in
dilute chloroform solution and as thin solid films. The
detailed absorption data including the absorption maximum
wavelength (kmax) in both solution and films, the absorption
edge (onset wavelength of the absorption peak, kedge) of the
polymer films, and optical bandgaps (Egopt) are summarized
in Table 2.
Except PDTS-TT, the absorption spectra of other three poly-
mers recorded from dilute chloroform solutions feature two
absorption bands in the range of 300–800 nm [Fig. 2(a)]:
the first one located at 330–390 nm can be assigned to
FIGURE 2 Absorption spectra of the four D-p-A copolymers in dilute chloroform solution (a) and solid film on a quartz plate (b).
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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TABLE 2 Optical and Electrochemical Properties of the Four Copolymers
Egopt
EHOMO
ELUMO
Copolymer
Solution ksmax (nm)
Film kfmax (nm)
kedge (nm)a
(eV)b
(eV)
(eV)
PBDT-AT
PDTS-AT
PBDT-TT
PDTS-TT
330, 568
388, 534
350, 568
556
354, 594
392, 552
356, 576
566
740
748
697
739
1.68
1.66
1.78
1.68
25.30
25.31
25.14
25.18
23.78
23.57
23.39
23.59
a
b
Absorption edge of the thin films.
Estimated from the absorption edge: Egopt 5 1,240/kedge
.
localized p–p* transition, and the second broader band from
400 to 750 nm in the long wavelength region corresponds to
ICT between donor and acceptor units. For the four poly-
mers the difference in position and shape of the absorptions
are mainly due to different structures of the p-bridges and
donors in the polymers. Compared with PDTS-AT, the
absorption spectrum of PBDT-AT is broader and the kmax of
ICT peak is red-shifted remarkably (568 vs. 534 nm), which
indicates that different electron donating strength of the
donor units (BDT vs. DTS) had a great influence on the
absorption. Similar phenomenon can be found to compare
PBDT-TT with PDTS-TT. Additionally, PDTS-TT displays a
red-shifted kmax of ICT peak relative to PDTS-AT, revealing
the effect of the different p-bridges on the optical properties
of the D-p-A copolymers. In contrast, PBDT-TT exhibits a
blue-shifted kmax compared with PBDT-AT, which mainly
results from the low polymerization degree of the former
with the significantly low molecular weight. The absorption
spectra of the polymer films are broader and obviously red-
shifted in comparison with those of their solutions [Fig.
2(b)], indicating intermolecular interactions existing in the
solid state. In a word, PBDT-AT possesses broader and red-
shifted absorption spectrum compared with the other three
polymers, indicating a higher Jsc could be obtained in solar
cells. The absorption edges (kedge) for solid films of the four
polymers varied from 697 to 748 nm, corresponding to
FIGURE 3 Cyclic voltammograms of the polymer films on plati-
num electrode in acetonitrile solution containing 0.1 mol L21
Bu4NPF6 at a scan rate of 0.1 V s21. The cyclic voltammogram
of ferrocene was also put in the figure for the potential calibra-
tion. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
FIGURE 4 (a) J–V curves of the PSCs based on PBDT-AT with
different donor/acceptor ratios; (b) J–V curves of the optimized
PSCs based on the four polymers with a certain amount of DIO
additive and treatment of methanol. [Color figure can be
viewed in the online issue, which is available at wileyonlineli-
brary.com.]
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TABLE 3 Photovoltaic Properties of the PSCs Based on the Copolymers:PC70BM with Different Fabrication Conditions, Under the
Illumination of AM1.5, 100 mW cm22
Active Layer
DIO
MeOH
Voc (V)
Jsc (mA cm22
)
FF (%)
PCE (%)
PBDT-AT:PC70BM 5 1:1
PBDT-AT:PC70BM 5 1:1.5
PBDT-AT:PC70BM 5 1:2
PBDT-AT:PC70BM 5 1:3
PBDT-AT:PC70BM 5 1:1
PBDT-AT:PC70BM 5 1:1
PBDT-AT:PC70BM 5 1:1
PBDT-AT:PC70BM 5 1:1
PDTS-AT:PC70BM 5 1:3
PDTS-AT:PC70BM 5 1:3
PDTS-AT:PC70BM 5 1:3
PBDT-TT:PC70BM 5 1:1.5
PBDT-TT:PC70BM 5 1:1.5
PBDT-TT:PC70BM 5 1:1.5
PDTS-TT:PC70BM 5 1:2
PDTS-TT:PC70BM 5 1:2
PDTS-TT:PC70BM 5 1:2
2
2
2
2
2
2
2
2
1
2
2
1
2
2
1
2
2
1
0.92
0.87
0.84
0.81
0.85
0.82
0.82
0.81
0.75
0.71
0.73
0.78
0.78
0.78
0.71
0.73
0.74
9.47
8.02
7.80
5.36
10.28
12.72
11.37
12.81
4.66
7.68
6.61
2.81
3.42
5.85
6.37
6.47
6.47
49.2
39.0
37.8
34.7
55.0
52.4
53.9
57.3
54.0
48.1
63.3
26.5
30.6
31.7
43.6
50.3
51.2
4.27
2.72
2.48
1.51
4.81
5.46
4.99
5.91
1.89
2.62
3.06
0.58
0.82
1.45
1.97
2.38
2.45
2
2
2
3%
5%
7%
5%
2
3%
3%
2
3%
3%
2
5%
5%
optical bandgaps (Egopt) from 1.78 to 1.66 eV, as shown in
Table 2. It should be noted that BDT-based polymer PBDT-
AT displayed lower bandgap (1.68 eV) than that of its ana-
logue without a p-bridge (1.78 eV).9
HOMO energy levels for PBDT-AT and PDTS-AT compared
with PBDT-TT and PDTS-TT. These phenomena indicate that
the different p-bridges have more significant effect on the
HOMO energy levels than that of the different donor units
in the D-p-A copolymers. In addition, the lower-lying HOMO
energy levels of PBDT-AT and PDTS-AT compared with
those of PBDT-TT and PDTS-TT is expected to give an
increase in Voc of PSCs. On the other hand, the LUMO energy
levels of the four polymers vary from 23.39 to 23.75 eV,
implying both the different donor units and p-bridges have
influence on the LUMO energy levels. It is well known that
an appropriate LUMO/HOMO energy level offset between a
polymer donor and a fullerene acceptor (e.g., PC70BM)
should be larger than 0.3–0.5 eV for efficient exciton dissoci-
ation on the donor/acceptor interfaces. The HOMO and
LUMO energy levels of PC70BM are 25.87 and 23.91 eV,
respectively.14 Obviously, the LUMO and HOMO energy levels
of the polymers are suitable for the application as donor
materials with PC70BM as acceptor in the BHJ PSCs. In addi-
tion, the HOMO energy levels of the polymers are lower
than 25.20 eV which is desirable for ambient stability to
oxidation.15
Electrochemical Properties
The HOMO and LUMO energy levels of conjugated polymers
are crucial for the application as photovoltaic materials in
PSCs, and they can be measured from the onset oxidation
and reduction potentials (Eox/red) in the cyclic voltammo-
grams of the polymers.12 Figure 3 shows the cyclic voltam-
mograms of the four polymer films on Pt electrode in a 0.1
mol L21 Bu4NPF6 acetonitrile solution. The results of the
electrochemical properties are also summarized in Table 2.
The cyclic voltammograms were recorded by using a Ag/
AgCl reference electrode, which was calibrated against the
redox potential of ferrocene/ferrocenium couple, whose
energy level was assumed to be 4.80 eV below the vacuum
level.13 Fc/Fc1 was measured to be 0.40 V versus Ag/AgCl,
thus EHOMO/LUMO 5 2e(Eox/red 1 4.40) (eV), where Eox/red is
the onset oxidation/reduction potential (vs. Ag/AgCl) of the
polymers.
Photovoltaic Properties
The Eox/red values of PBDT-AT, PDTS-AT, PBDT-TT, and
PDTS-TT are 0.90/20.65, 0.91/20.83, 0.74/21.01, and
0.78/20.81 V versus Ag/AgCl, respectively. Based on the
Eox/red values of the polymers, the EHOMO/LUMO values of
PBDT-AT, PDTS-AT, PBDT-TT, and PDTS-TT were calculated
to be 25.30/23.75, 25.31/23.57, 25.14/23.39, and
25.18/23.59 eV, respectively, which are listed in Table 2.
The HOMO energy levels of PBDT-AT and PDTS-AT are very
close and the same results can be found from PBDT-TT and
PDTS-TT. However, there is a considerable difference in the
To investigate the effect of donor unit and p-conjugated
bridge on the photovoltaic properties of the D-p-A copoly-
mers, BHJ PSCs with a configuration of ITO/PEDOT:PSS/poly-
mer:PC70BM/Ca/Al were fabricated. Figure 4 shows the
current density–voltage (J-V) curves of the PSCs under the
illumination of AM1.5G, 100 mW cm22. The corresponding
open-circuit voltage (Voc), short-circuit current density (Jsc),
fill factor (FF), and PCE of the devices are summarized in
Table 3.
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First, the blend ratio of the polymer donor and PC70BM
acceptor was optimized. For example, when the mixing ratio
of PBDT-AT and PC70BM changed from 1:1 to 1:3, the PCE of
the device continuously decreased from 4.27% to 1.51%
[Fig. 4(a); Table 3], due to the decrease of both Voc and FF of
the PSCs. It was observed that an increase in the PC70BM
content resulted in a slight decrease in the Voc, which has
been observed for other donor polymers.16 The reduction in
the Voc correlated with the decrease in the energy of the
charge transfer (CT) states with increasing PC70BM concen-
trations.16d Therefore, the optimal blend ratio for this poly-
mer was determined to be around 1:1. The blend ratio of
the other three polymers was also optimized. It was found
that the different polymers have their own optimal blend
ratio and the determined optimal blend ratios are 1:3, 1:1.5,
and 1:2, for PDTS-AT, PBDT-TT, and PDTS-TT, respectively.
Next, the effect of a processing additive, such as 1,8-diio-
dooctane (DIO), upon the photovoltaic properties of the PSCs
was explored, the results are shown in Table 3. We take
PBDT-AT as an example, when DIO was added as the proc-
essing additive, the Jsc and FF values can be improved signifi-
cantly, resulting in remarkably improvement of PCEs even
though the Voc values dropped in some extend. The decrease
of the Voc values could result from increased charge carrier
recombination due to the changed interpenetrating network
by DIO additive treatment. Moreover, the concentration of
the DIO additive was optimized. For the PBDT-AT–based
PSCs, the PCE was improved up to 5.46% with 5 vol % DIO
additive, with a Voc of 0.82 V, a Jsc of 12.72 mA cm22 and a
FF of 52.4%. For other polymers, the photovoltaic perform-
ance was also improved with a certain amount of DIO addi-
tive. The PCEs of the optimized PSCs based on PDTS-AT,
PBDT-TT, and PDTS-TT were 2.62%, 0.82%, and 2.38%,
respectively (see Table 3).
originate from enhanced charge transport, or improved effi-
ciency of charge carrier collection, which can all result from
the creation of an optimal morphology under the treatment
of DIO additive and methanol. The morphology will be dis-
cussed in detail below.
Voc value of the PSC based on PBDT-AT is higher than those
of the PSCs based on PBDT-TT and PDTS-TT, which can be
readily understood from the lower-lying HOMO energy level
of PBDT-AT (Table 2). But, the relatively lower Voc of the PSC
based on PDTS-AT with a low-lying HOMO energy level could
be due to poorer morphology of its active layer or other rea-
sons.18 The highest Jsc values of PSCs based on these poly-
mers increased from 5.85 mA cm22 for PBDT-TT to 6.47 mA
cm22 for PDTS-TT to 7.68 mA cm22 for PDTS-AT to 12.81
mA cm22 for PBDT-AT, which are consistent with the broad-
ened absorption of the corresponding polymers. It should be
noted that the Jsc based on PDTS-TT is obvious lower than
that of PBDT-AT even though they have the similar broad
spectra, which should be ascribed to the different molecular
weight of the polymers. The lower molecular weight PDTS-TT
showed lower Jsc and the higher molecular weight PBDT-AT
demonstrated higher Jsc (see Fig. 2; Tables 1 and 2).
Figure 5 displays the external quantum efficiency (EQE)
curves of the PSC. The PSCs based on these polymers exhibit
broad EQE responses ranging from 300 to 750 nm with max-
imum EQE values of 67% at 555 nm, 44% at 470 nm, 42%
at 500 nm, and 44% at 494 nm for PBDT-AT, PDTS-AT,
PBDT-TT, and PDTS-TT, respectively. Compared with the
absorption spectra of polymer films [Fig. 2(b)], the signifi-
cantly broadened EQE responses in the visible region can be
attributed to both the intrinsic absorption of the polymers
and PC70BM. The EQE results are consistent with the Jsc val-
ues of the corresponding PSCs, and PBDT-AT showed the
highest EQE values corresponding to the highest Jsc value of
the PSC based on PBDT-AT.
Inspired by recent work on post solvent treatment,17 we
tried to employ a polar solvent methanol to further optimize
the photovoltaic performance of PSCs with the following pro-
cedure: (i) spin-coating the active layer and dried in inert
atmosphere; (ii) methanol was added on top of the active
layer and left for a very short time (such as 10 sec); (iii)
methanol was removed by spin-coating at high speed (such
as 2500 rpm); (iv) the metal cathode (Ca and Al) was evapo-
rated on the active layer. The results are shown in Figure
4(b) and Table 3. It can be seen that after the solvent treat-
ment the photovoltaic performance of all the copolymers
was enhanced. For PBDT-AT, the PCE was further improved
up to 5.91% with an increased Jsc from 12.72 to 12.81 mA
cm22, and a FF from 52.4% to 57.3%. For PDTS-AT, the PCE
increased from 2.62% to 3.06%, with a Voc from 0.71 to 0.73
V and a largely increased FF from 48.1% to 63.3%. For
PBDT-TT, the PCE achieved 1.45%, mainly due to an obvi-
ously improved Jsc from 3.42 to 5.85 mA cm22. As for PDTS-
TT, there was a slightly increased PCE up to 2.45%. More-
over, the PCE of 5.91% for PBDT-AT is significantly higher
than that of the analogy polymer (2.18%)9 without p-bridge,
indicating the importance of a p-bridge to achieve high per-
formance D-p-A copolymers. The improved efficiency may
FIGURE 5 Typical EQE curves of solar cells of the four poly-
mers with methanol treatment. [Color figure can be viewed in
the online issue, which is available at wileyonlinelibrary.com.]
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S1 exhibit no obvious diffraction peaks, which mean these
four polymer films were amorphous. This could be a reason
for relatively lower FF for the PSCs based on these polymers.
Hole mobility is a very important parameter for conjugated
polymer donor photovoltaic materials. Here, the hole mobil-
ity of the polymer/PC70BM blend film with the optimal D/A
ratio (w/w) was measured by the space-charge limited cur-
rent (SCLC) method on a hole-only device with a structure
of ITO/PEDOT:PSS/polymer:PC70BM/Au and estimated
through the Mott–Gurney equation.19 Figure 6 shows the
ln(JL3/V2) versus (V/L)0.5 plot of the blend films. The hole
mobilities of the polymers calculated are 4.52 3 1023, 1.84
3 1025, 1.23 3 1024, and 4.09 3 1024 cm2 V21 s21 for
PBDT-AT, PDTS-AT, PBDT-TT, and PDTS-TT, respectively. It
should be noted that the hole mobility of polymer:PC70BM
blend was enhanced dramatically after treatment with meth-
anol. For example, the hole mobility of PBDT-AT-based blend
treated with methanol was increased by three times, from
4.52 3 1023 to 1.83 3 1022 cm2 V21 s21. The improved
hole mobility could be ascribed to the better nanoscale mor-
phology of the active layer after methanol treatment. The
higher hole mobility could contribute in part to the higher
Jsc and FF values of the devices with methanol treatment
(see Table 3).17e,f The higher hole mobility of the PBDT-AT–
based blend film could be related to its higher molecular
FIGURE 6 Plots of ln(JL3 V22) versus (V0.5 L20.5) of the device
ITO/PEDOT:PSS/blended film/Au for the measurement of hole
mobility. [Color figure can be viewed in the online issue, which
is available at wileyonlinelibrary.com.]
To investigate crystalline behavior of the polymers, we made
the X-ray diffraction (XRD) analysis of the four polymer films,
as shown in Figure S1 in Supporting Information. The XRD
patterns of the four films in Supporting Information Figure
FIGURE 7 AFM height images of (a) PBDT-AT:PC70BM (1:1), (b) PBDT-AT:PC70BM (1:1, 5 vol % DIO), (c) PBDT-AT:PC70BM (1:1, 5
vol % DIO, treated with methanol), (d) PDTS-AT:PC70BM (1:3), (e) PDTS-TT:PC70BM (1:1.5), and (f) PDTS-TT:PC70BM (1:2) blend
films. The imaging size is 5 lm 3 5 lm for each panel.
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weight (see Table 1).4a,20 While for the other three polymers
with relatively lower molecular weight, the hole mobilities of
the polymers with TT as the p-bridge (PBDT-TT and PDTS-
TT) are one order of magnitude higher than that of the poly-
mer with AT as the p-bridge (PDTS-AT), which indicates that
TT p-bridge in the D-p-A copolymers is beneficial to higher
hole mobility of the polymers because of its planar and
larger molecular plane.6f
the donor units in the D-p-A polymers, and the polymers
with TT p-bridge show up-shifted HOMO energy level than
that of the polymers with AT p-bridge. PCE of the PSCs based
on PBDT-AT with methanol treatment reached 5.91%, which
is greatly improved in comparison with the corresponding
polymer without the AT p-bridge. The effect of methanol
treatment on the enhancement of photovoltaic performance
is revealed to originate from the increased carrier transport
and the formation of better nanoscaled interpenetrating net-
work. This work provides a good example for tuning absorp-
tion properties, energy levels, charge transport and
photovoltaic properties of the D-p-A copolymers by appropri-
ate combination of the donor units and p-bridges. Most
importantly, this study clearly proves that BDD is a promis-
ing acceptor unit to construct D-A copolymers for high-
performance photovoltaic materials.
To explore the effect of the morphology of active layers on
the photovoltaic performance of the four D-p-A copolymers,
we observed the surface morphologies of the blend films
using atomic force microscopy (AFM). Figure 7 gives the
AFM height images of the active layer surfaces. It can be
seen that the surface of the active layers of polymer:PC70BM
without DIO additive is very smooth, with a root-mean-
square (RMS) of 0.878, 0.661, 0.685, and 0.505 nm for
PBDT-AT, PDTS-AT, PBDT-TT, and PDTS-TT, respectively.
The active layer of PBDT-AT showed a relative rougher sur-
face compared with the other three polymers, which sug-
gests that relative larger phase separation is more favorable
for charge transport. After adding the DIO additive (e.g., 5
vol % for PBDT-AT blend film), the surface roughness of the
active layer based on PBDT-AT increased to a RMS of 2.790
nm [see Fig. 7(b)]. Moreover, after methanol treatment the
surface roughness of the blend film further increased to a
RMS of 4.59 nm, and the surface demonstrated a more uni-
form interpenetrating network of PBDT-AT and PC70BM [see
Fig. 7(c)]. The long-range interpenetrating network in the
active layer of PBDT-AT/PC70BM after methanol treatment
should be beneficial to the exciton dissociation and charge
carriers transport. The appropriate surface roughness at
about 4–5 nm and the improved interpenetrating network of
the polymer/PC70BM blend should be responsible for the
superior photovoltaic performance of the PBDT-AT-based
PSCs with methanol treatment (see Table 3).
EXPERIMENTAL
Measurements and Characterization
1H NMR spectra and 13C NMR spectra were measured on a
Bruker DMX-400 spectrometer with CDCl3 as the solvent and
trimethylsilane as the internal reference. UV–visible (UV–vis)
absorption spectra were measured on a Hitachi U-3010 UV–
vis spectrophotometer. Mass spectra (MS) were recorded on
a Shimadzu spectrometer and MALDI-TOF-MS spectra were
determined on a Bruker BIFLEX III mass spectrometer. TGA
was conducted on a Perkin–Elmer TGA-7 thermogravimetric
21
ꢁ
analyzer at a heating rate of 20 C min and under a nitro-
gen flow rate of 100 mL min21. Molecular weights of the
polymers were measured by GPC method on Waters 515-
2410 with polystyrenes as standard and THF as an eluent.
The electrochemical cyclic voltammetry was performed on a
Zahner IM6e Electrochemical Workstation, with a Pt disk
coated with the polymer film, Pt wire, and Ag/AgCl electrode
as the working electrode, counter electrode, and reference
electrode, respectively, in a 0.1 mol L21 tetrabutylammonium
hexafluorophosphate (Bu4NPF6) acetonitrile solution. The
morphology of blend films was measured using an AFM
(Digital Instrument Multimode Nanoscope IIIA) with the tap-
ping mode.
CONCLUSIONS
In summary, four D-p-A type copolymers PBDT-AT, PDTS-AT,
PBDT-TT, and PDTS-TT, based on the same BDD acceptor
unit with different donor units and p-bridges, were designed
and synthesized via a Pd-catalyzed Stille-coupling method.
Two kinds of donor units (BDT and DTS) and p-bridges (AT
and TT) were employed to fully investigate the effect of the
different donor units and p-bridges upon the optical and
electrochemical properties, hole mobilities, and photovoltaic
performance of the copolymers. These D-p-A copolymers
possessed broad absorption (ranging from 300 to 800 nm)
and lower energy bandgaps (from 1.66 to 1.78 eV). The poly-
mers with BDT as the donor unit showed broader and red-
shifted absorption spectra compared with the counterparts
with DTS as the donor unit. And the polymers with TT p-
bridge gave red-shifted absorption spectra than that of the
polymers with AT p-bridge although the molecular weight of
the polymers with TT p-bridge is lower. The results of elec-
trochemical measurements revealed that the p-bridges have
more obvious effect on the HOMO energy levels than that of
PSC Device Fabrication and Characterization
The PSC devices were fabricated with a structure of ITO/
PEDOT:PSS/polymer:PCBM/Ca/Al. The patterned ITO glass
was precleaned in an ultrasonic bath of acetone and isopro-
panol and treated in an ultraviolet-ozone chamber (Jelight
Co.) for 30 min. A thin layer of PEDOT:PSS [poly(3,4-ethyle-
nedioxythiophene):poly(styrenesulfonate)] was spin-cast on
precleaned ITO-coated glass from a PEDOT:PSS aqueous
solution (Baytron P VP AI 4083 from H. C. Starck) at 3000
rpm and dried subsequently at 150 ꢁC for 30 min in air,
then the device was transferred to a glovebox, where a blend
solution of the polymer and PC70BM with the polymer con-
centration of 10 mg mL21 was spin-coated onto the
PEDOT:PSS layer. Finally, a Ca/Al metal top electrode was
deposited in vacuum onto the active layer at a pressure of
about 5 3 1025 Pa. The active area of the device was about
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4 mm2. The current density–voltage (J–V) characteristics
into a mixture of ice water and 1 M hydrochloric acid and
extracted with dichloromethane. The organic layer was
washed with brine and dried over anhydrous MgSO4, and
the solvent was removed under vacuum. The crude product
was purified through the silica gel column with petroleum
ether/dichloromethane (2:1 by volume) as eluent to give
were measured on
a computer-controlled Keithley 236
Source–Measure Unit. A xenon lamp (150 W) coupled with
AM 1.5 solar spectrum filter was used as the light source,
and the optical power at the sample was 100 mW cm22
.
EQE spectrum was measured by a Stanford Research Sys-
tems model SR830 DSP lock-in amplifier coupled with WDG3
monochromator and a 150 W Xenon lamp. The hole mobility
was calculated by fitting the dark J–V curves for the hole-
only devices with SCLC model at low voltages, in which the
current density is given by J 5 9e0erlV2/8L3exp[0.891c(V/
L)0.5], where e0er represents the permittivity of the material,
l is the mobility, c is the field activation factor, and L the
thickness of the active layer. The applied bias voltage is cor-
rected for the built-in potential so that V 5 Vapplied 2 Vbi.
1
compound 3 as a yellow solid (0.63 g, yield 12.9%). H NMR
(400 MHz, CDCl3): d (ppm) 7.21 (s, 1H), 2.78–2.76 (d, 2H, J
5 6.68 Hz), 1.60–1.54 (m, 1H), 1.34–1.19 (m, 8H), 0.87–0.82
(m, 6H). MALDI-TOF-MS m/z: [M1] calculated for
C18H18Br2O2S2, 491.9; found, 491.7.
2-(2-Ethylhexyl)-5,7-bis(4-hexylthiophen-2-yl)benzo[1,2-
b:4,5-c’]dithiophene-4,8-dione (4a). A solution of com-
pound 3 (1.57 g, 3.20 mmol) and 4-hexyl-2-(tributylstan-
nyl)thiophene (4.16 g, 8.00 mmol) in freshly distilled toluene
(35 mL) was degassed. Under an argon atmosphere the
Pd(PPh3)4 (220 mg, 0.19 mmol) was added. And then the
mixture was heated and refluxed for 24 h, toluene was
removed by vacuum distillation. The crude product was puri-
fied through the silica gel column with petroleum ether/
dichloromethane (4:1 by volume) to give compound 4a as a
wine red solid (1.45 g, yield 68%). 1H NMR (400 MHz,
CDCl3): d (ppm) 7.73–7.71 (d, 2H, J 5 8.80 Hz), 7.33 (s, 1H),
7.14 (s, 2H), 2.83 (d, 1H, J 5 6.44 Hz), 2.65 (t, 4H, J 5 7.08
Hz), 1.66–1.62 (m, 5H), 1.36–1.29 (m, 20H), 0.94–0.88 (m,
12H). MALDI-TOF-MS m/z: [M1] calculated for C38H48O2S4,
666.3; found, 666.7.
Synthesis of Monomers and Polymers
3,4-Thiophenedicarboxylic acid, 3-hexylthiophene, oxalyl
chloride, NBS, 1-bromo-2-ethylhexane, 1-bromo-2-hexylde-
cane, TT, anhydrous AlCl3, and Pd(PPh3)4 were purchased
from J&K Chemical or Alfa Aesar. Toluene was distilled from
sodium benzophenone under nitrogen before use. All other
reagents and solvents used in this work were commercially
purchased and used without further purification. All chro-
matographic separations were carried out on silica gel (200–
300 mesh). Compounds 2-(2-ethylhexyl)thiophene,21 2-(2-
hexyldecyl)thiophene,21 2-(tributylstannyl)thieno[3,2-b]thio-
phene,21 tributyl(4-hexylthiophen-2-yl)stannane,21 monomers
M3,7b M4,7b and M511 were synthesized according to the
procedure reported in the literatures. The detailed synthetic
processes of other compounds are as follows.
2-(2-Ethylhexyl)-5,7-bis(thieno[3,2-b]thiophen-2-
yl)benzo[1,2-b:4,5-c’]dithiophene-4,8-dione (4b). This
compound was obtained as a red solid (yield 57%) from the
reaction of compound 3 and 2-(tributylstannyl)thieno[3,2-
b]thiophene according to the similar procedure described for
2,5-Dibromothiophene-3,4-dicarboxylic acid (1). Thio-
phene-3,4-dicarboxylic acid (5.00 g, 29.1 mmol) and glacial
acetic acid (50.0 mL) were added to a 250 mL flask with a
stirring bar. Bromine (9.00 mL, 174 mmol) was added drop-
wise. The mixture was stirred overnight. Aqueous sodium
bisulfate solution was added until the reddish color disap-
peared. The mixture was filtered and washed with 50 mL
water, and compound 1 was afforded as a gray solid (9.00 g,
yield: 75%). MS m/z: [M1] calculated for C6H2Br2O4S, 329.8;
found, 330.
1
the synthesis of compound 4a. H NMR (400 MHz, CDCl3): d
(ppm) 8.20 (s, 1H), 8.17 (s, 1H), 7.50–7.48 (m, 2H), 7.28 (s,
1H), 7.25–7.24 (m, 2H), 2.83–2.78 (m, 2H), 1.65–1.62 (m,
1H), 1.38–1.29 (m, 8H), 0.94–0.87 (m, 6H). 13C NMR (100
MHz, CDCl3, d/ppm): 175.84, 174.21, 155.59, 145.46, 144.38,
144.26, 144.12, 142.50, 139.53, 139.44, 134.40, 134.30,
130.49, 130.10, 127.36, 124.48, 123.60, 123.47, 119.44,
41.39, 34.81, 32.37, 28.81, 25.57, 22.93, 14.09, 10.79.
MALDI-TOF-MS m/z: [M1] calculated for C30H24O2S6, 609.0;
found, 609.2.
2,5-Dibromothiophene-3,4-dicarboxylic acid chloride (2).
Oxalyl chloride (5.30 mL, 60.5 mmol) was slowly added to
compound 1 (5.00 g, 15.2 mmol) and DMF (one drop) in dry
dichloromethane (100 mL). The mixture was stirred over-
night under room temperature. The volatiles were removed
in vacuum, and a brown product as compound 2 was
directly used in the next step without further purification.
5,7-Bis(5-bromo-4-hexylthiophen-2-yl)-2-(2-ethylhexyl)-
benzo[1,2-b:4,5-c’]dithiophene-4,8-dione (M1). NBS (0.65
g, 3.60 mmol) in DMF (15 mL) was added dropwise to a solu-
tion of compound 4a (1.20 g, 1.80 mmol) in DMF (35 mL),
which was cooled in an ice bath. After complete addition, the
reaction mixture was allowed to warm to room temperature
and stirred for overnight. The reaction mixture was poured
into water and extracted with chloroform. The organic layers
were combined, washed with brine, and dried with anhydrous
MgSO4. Further purification was performed through silica gel
chromatography using petroleum ether/dichloromethane (6:1
by volume) to give monomer M1 (1.27 g, yield 86%) as a deep
red solid. 1H NMR (400 MHz, CDCl3): d (ppm) 7.51 (s, 1H),
5,7-Dibromo-2-(2-ethylhexyl)benzo[1,2-b:4,5-c’]dithio-
phene-4,8-dione (3). To a stirred solution of compound 3
(3.65 g, 10.0 mmol) and 2-(2-ethylhexyl)thiophene (2.16 g,
11.0 mmol) in dry dichloromethane, anhydrous AlCl3 (3.00 g,
22.5 mmol) was added in small portions at 0 ꢁC. The mix-
ꢁ
ture was stirred at 0 C for 1 h, and then at room tempera-
ture for additional 5 h. The reaction mixture was poured
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7.50 (s, 1H), 7.33 (s, 1H), 2.83 (d, 2H, J 5 6.48 Hz), 2.60 (t, 4H,
J 5 6.81 Hz), 1.63–1.58 (m, 5H), 1.34–1.29 (m, 20H), 0.91–
0.88 (m, 12H). 13C NMR (100 MHz, CDCl3, d/ppm): 175.77,
174.19, 155.77, 145.13, 144.06, 143.19, 142.81, 142.43,
132.21, 132.17, 132.50, 129.45, 129.11, 124.45, 116.49, 41.38,
34.83, 32.39, 31.62, 29.72, 29.50, 29.00, 28.82, 25.55, 22.93,
22.62, 14.09, 10.79. MALDI-TOF-MS m/z: [M1] calculated for
Poly{4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-
b’]dithiophen-alt-2-(2-ethylhexyl)-5,7-bis(thieno[3,2-b]thio-
phen-2-yl)benzo[1,2-b:4,5-c’]dithiophene-4,8-dione} (PBDT-
TT). PBDT-TT was synthesized by following the same proce-
dure for PBDT-AT. M2 (153 mg, 0.20 mmol) and M4 (225
mg, 0.20 mmol) were used as starting materials. Finally,
PBDT-TT was obtained as a black solid (197 mg, yield
69%). 1H NMR (CDCl3, 400 MHz): d (ppm) 7.80–7.78 (br,
2H), 7.76–7.40 (br, 2H), 7.38–7.36 (br, 2H), 7.28 (br, 2H),
6.73 (br, 1H), 2.85 (br, 9H), 1.75–0.78 (br, 74H). ELEM. ANAL.
for(C80H94O2S10)n Calc: C, 68.13; H, 6.86; Found: C, 66.94; H,
6.67.
C38H48Br2O2S4, 824.1; found, 824.6.
5,7-Bis(5-bromothieno[3,2-b]thiophen-2-yl)-2-(2-ethyl-
hexyl)benzo[1,2-b:4,5-c’]dithiophene-4,8-dione (M2). This
compound was obtained as a dark red solid (yield 85%)
from the reaction of compound 4b and NBS according to the
similar procedure described for the synthesis of monomer
M1. 1H NMR (400 MHz, CDCl3): d (ppm) 8.04 (s, 1H), 8.00
(s, 1H), 7.23 (s, 2H), 7.22 (s, 1H), 4.15–4.09 (m, 1H), 2.77–
2.76 (d, 2H, J 5 6.80 Hz), 1.56–1.23 (m, 8H), 0.92–0.88 (m,
6H). 13C NMR (100 MHz, CDCl3, d/ppm): 175.49, 173.85,
155.71, 145.10, 143.86, 143.62, 143.33, 141.76, 141.71,
139.56, 139.48, 133.80, 133.72, 130.48, 130.11, 124.31,
122.87, 122.75, 122.07, 177.16, 41.31, 34.76, 32.36, 28.78,
25.52, 22.97, 14.13, 10.76. MALDI-TOF-MS m/z: [M1] calcu-
lated for C30H24Br2O2S6, 765.8; found, 765.3.
Poly{4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30-d]silole-
5,50-diyl-alt-2-(2-ethylhexyl)-5,7-bis(thieno[3,2-b]thio-
phen-2-yl)benzo[1,2-b:4,5-c’]dithiophene-4,8-dione}
(PDTS-TT). PDTS-TT was synthesized by following the
same procedure for PBDT-AT. M2 (200 mg, 0.26 mmol) and
M5 (190 mg, 0.26 mmol) were used as starting materials.
Finally, PDTS-TT was obtained as a black solid (93 mg,
yield 35%). 1H NMR (CDCl3, 400 MHz): d (ppm) 8.02 (br,
2 H), 7.43 (br, 1H), 7.08 (br, 2H), 6.72–6.70 (br, 2H), 2.96–
2.69 (br, 6H), 1.54–0.87 (br, 45H). ELEM. ANAL. for
(C54H58O2S8Si)n Calc: C, 63.24; H, 5.90; Found: C, 62.52; H,
5.87.
Poly{4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-
b’]dithiophen-alt-2-(2-ethylhexyl)-5,7-bis(4-hexylthiophen-2-
yl)benzo[1,2-b:4,5-c’]dithiophene-4,8-dione} (PBDT-AT). The
polymerization was performed by a Stille coupling reaction. In
a 50 mL flask, M1 (182 mg, 0.22 mmol) and M3 (201 mg, 0.22
mmol) were dissolved in 10 mL toluene, and the solution was
flushed with argon for 15 min, then 30 mg Pd(PPh3)4 was
added into the solution. The mixture was again flushed with
argon for 20 min. The reaction mixture was heated to reflux
for 24 h. The reaction mixture was cooled to room tempera-
ture and added dropwise to 200 mL methanol. The precipitate
was collected and further purified by Soxhlet extraction with
methanol, hexane, and chloroform in sequence. The chloro-
form fraction was concentrated and added dropwise into
methanol. Finally, the precipitates were collected and dried
under vacuum overnight to get PBDT-AT as a purple solid
(219 mg, yield 80%). 1H NMR (CDCl3, 400 MHz): d (ppm) 7.78
(br, 3H), 7.43 (s, 1H), 7.38 (br, 3H), 6.94 (br, 2H), 2.90–2.85
(br, 10H), 1.73–1.23 (br, 43H), 1.17–1.08 (br, 24H). ELEM. ANAL.
for (C72H86O2S8)n Calc: C, 69.63; H, 7.14; Found: C, 69.17; H,
7.18.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundation of China (21004050, 50933003), the General Pro-
gram of the Education Department of Hunan Province
(No.YB2011B030), and the Innovation Group and Xiangtan
Joint Project in Hunan Natural Science Foundation (12JJ7002).
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